Just to confirm, I had exactly the same issue with ARC from day one:
As soon as I had to reboot my router or just the modem for some reason, the connection via UPnP regularly stopped working afterwards.
A core reboot fixed this with UPnP and ARC working again. I did not try to reboot only the server.
(That’s why I configured a port forwarding rule for my router instead of UPnP, so everything runs perfectly now without any interruptions or failures.)
My router: ASUS RT-AX86U
Modem in bridge mode: Huawei CPE Pro 5g
Core: ROCK, NUC i7